1. Field of the Invention
The present invention relates to a spread spectrum communication system and, more particularly, to a spread spectrum communication system which uses communication signals modulated based on the spread spectrum.
2. Description of the Related Art
A spread spectrum communication system permits a plurality of users to perform communication while sharing one wide frequency band on a time and spatial basis, has a large system capacity per bandwidth and is resistive to interference. Thus, such communication system is receiving attention as a solution to the existing excessive concentration of electric waves. In such a spreading spectrum communication system, a specific spreading code is assigned to each user to discriminate signals for that user and spread spectrum is carried out by using the spreading code on the transmitter side. Inverse spread spectrum (despreading) is conducted on the receiver side by using a copy of the spreading code used in the spreading spectrum.
FIG. 1 shows an example of a modulating circuit on the transmitter side in the spread spectrum communication system. A transmission signal and a spreading code, which is specific to the user who uses a concerned terminal and generated from a spread code generator 1, are supplied to a mixer 2. The mixer 2 performs a spreading process on the transmission signal using the spreading code, and sends the resultant signal to a modulator 3. The modulator 3 executes spread-modulation to produce a carrier frequency signal and sends it to a power amplifier 4. The power amplifier 4 amplifies the carrier frequency signal to a proper level and sends the amplified signal to an antenna 5.
FIG. 2 shows an example of a demodulating circuit on the receiver side in this spread spectrum communication system. A carrier frequency signal, which is received by an antenna 11 and amplified by a radio frequency amplifier 12, and a local oscillation signal output from a first local oscillator 13 are supplied to a first mixer 14 which mixes both signals to produce a first intermediate frequency signal. The first intermediate frequency signal is put through a first bandpass filter 15 to become a first intermediate frequency signal of the desired band. This first intermediate frequency signal is sent to a second mixer 16. The second mixer 16 also receives a local spread signal that is obtained by mixing a local oscillation signal output from a second local oscillator 17 and a spreading code output from a spreading code generator 18, which specifies the receiving user, by a local mixer 19. The second mixer 16 performs a mixing process (despreading) on the first intermediate frequency signal and the spread signal to produce a desired narrow band signal (second intermediate frequency signal). This second intermediate frequency signal is sent to a second bandpass filter 20 to eliminate the unnecessary signal component, yielding a desired signal. The desired signal is then sent to a demodulator 21, which demodulates the desired signal to extract desired information by means of a conventional scheme (digital demodulation or analog demodulation such as FM demodulation).
To properly carry out communication between the transmitter and the receiver, this spread spectrum communication system requires that the spreading code set on the transmitter side should coincide with the spreading code set on the receiver side, and that, in addition, those spreading codes should differ from those for the other channel users who also use this communication system. Besides, in the spread spectrum communication system, a spreading code which is used in wide band services such as image transmission, high speed data transmission etc. has a longer code period than that of a spreading code which is used in narrow band services such as voice transmission, low speed data transmission, etc.
In the spread spectrum communication system, individual services need their spreading codes assigned uniquely to individual users. Even for the transmission of signals of different bandwidths, the spreading codes are also specific to the individual users. If the number of users and/or the number of services is increased, the managing system for the users and services which are managed together as well as the management of those spreading codes becomes complicated. Thus, the system design becomes complex as a consequence.